Explosive charges used by the oil industry in working deep wells are subjected to highly adverse conditions. Such charges and, obviously, the detonators employed to initiate explosion of the charges, must be capable of withstanding and remaining reliably operative in temperatures from 204.degree. to 315.degree. C. (400.degree. to 600.degree. F.) for periods up to 200 hours. Prior to this invention there have been no percussion initiated detonators capable of reliable operation above 218.degree. C. (425.degree. F.) for periods up to 200 hours.
Detonators designed for use in less harsh environments have been susceptible to two types of failures when subjected to prolonged high temperature environments. One type of failure results in premature detonator initiation because the temperature encountered causes ignition of the sensitive primer charge in the detonator. This causes detonation of the main explosive charge before it can be brought to the desired location in the well. The other type of failure renders the detonator useless because one or more of the charges therein disintegrate at high temperatures and provide insufficient energy to detonate the main explosive charge.
To prevent premature initiation failure the detonator must contain charges which are not subject to ignition when merely subjected to the elevated temperature. But, even with less temperature sensitive charges being used, the configuration of the detonator and its charges must be such as to generate sufficient explosive energy to detonate the main explosive charge. Known detonators do not possess this capability after long term exposure to temperatures above 218.degree. C. (425.degree. F.).
The prevention of detonator charge disintegration failure imposes further configuration requirements for the detonator. It has been known for some time that explosive charges of the type commonly used in detonators break down into gasseous components when subjected to elevated temperatures. It has also been recognized that unless provision is made for escape of these gasses from the body of the charge the presence of the gasses themselves contribute to and hasten further deterioration of the charge. It has, therefore, been proposed to provide a vent, or escape route, in the detonator for gasses emanating from the output charge of the detonator. Thus, although deterioration of the detonator charge may commence when the detonator is subjected to some predetermined high temperature, if provision is made for venting the out-gasses the rate of deterioration can be slowed to give the detonator a longer useful life at that elevated temperature. In other words, even after some extended period of time and with some limited deterioration of the charge, the charge will still contain sufficient energy to detonate the main explosive charge. A detonator with a vented output charge has been manufactured and sold by the assignee of this application, Unidynamics Phoenix Inc., of Phoenix, Ariz., under the brand name Geo Vann. So far as is known, no prior detonators were configured to vent any of the charges other than the output charge.